Abstract Generation of robust Th17 immune responses required for clearance of certain pathogens depends on promoting Th17 while reciprocally inhibiting Treg formation. Whereas, effective prevention of Th17-mediated autoimmunity such as EAE depends on inhibiting pathogenic Th17 while reciprocally promoting Treg formation. However, little is known about the mechanisms responsible for coordination of Th17 and Treg differentiation. Our preliminary results demonstrated that PKC-theta is a critical checkpoint for reciprocal Th17 and Treg differentiation. The proposed studies will investigate the function of PKC- theta and RORyt in the reciprocal Th17 and iTreg differentiation. Based on the knowledge learned from the studies, we expect to develop PKC-8-based treatments for prevention of EAE, an animal model of multiple sclerosis. It is expected that such treatments will have a broader applicability in the prevention of Th17-mediated autoimmunity. In addition, the proposed research has significance to basic T cell biology, as it is expected to reveal novel molecular mechanisms for PKC- theta -mediated TCR signals in the coordination of Th17 and iTreg differentiation.